notochord

Characteristic of chordates, a stiff rod
of tissue along the back of the body which is the first stage in the development
of a flexible internal skeleton.

All chordates possess a notochord at some time during their lives. Though
replaced by cartilage or bone
in the adult vertebrate and absent in
the adults of other chordate groups, e.g., tunicates, it is well developed
in the embryos and larvae of all these groups, confirming evolutionary relationships
within the class.